The present invention relates to orthovoltage x-ray machines for radiation therapy and in particular to a filter for improving the sharpness of radiation beams generated by such machines.
Many stereotactic radiosurgery (SRS) and stereotactic body radiation therapy (SBRT) treatments require highly conformal dose distributions with sharp dose gradients at the target periphery. Delivering a more homogeneous dose to the target volume has been associated with reduced adverse effects and toxicities. Delivering a precise dose to the target volume also allows for the prescription to a higher isodose volume without excessive dose to nearby critical structures.
One method for achieving sharp dose distributions is through modulation of the SRS beam combined with the use of low energies. Previous work with 6 megavolt SRS pencil beams has used sets of concentric hypodermic tubing of varying lengths and diameters inserted into standard SRS cone collimators to increase profile uniformity within the irradiated volume, as well as to increase the steepness of the dose gradient on the periphery. See E. T. Bender, “Increasing dose gradient and uniformity in small fields using modulation: theory and prototypes for cone-based stereotactic radiosurgery,” Med. Phys. 41, 5, 051706-1-051706-7525 (2014) hereby incorporated by reference.
The sharpness of the pencil beam's dose gradient is defined by a combination of geometric penumbra and radiologic penumbra. The geometric penumbra is caused by the finite size of the x-ray focal spot of the x-ray tube interacting with the edges of a downstream collimator. Generally, the smaller the x-ray focal spot and the closer the collimation to the treatment volume, the smaller the geometric penumbra. The radiologic penumbra is caused by scattering within the tissue of the patient and is largely indifferent to the geometry of the x-ray system.
For standard SRS fields, the radiological penumbra is the dominating component of the overall penumbra. Reduction of beam energy from the standard megavoltage range into the orthovoltage energy range offers a dosimetric benefit by reducing the range of the secondary electrons generated by the x-rays and thus shrinking the radiological penumbra. Unfortunately, the focal spot sizes for orthovoltage units are larger than those for standard 6 MV linear accelerators increasing the geometric penumbra. This geometric penumbra may be minimized to some extent by the use of collimators near the volume to be irradiated.